The main problem with carps (koi?), goldfish, and plecos (these can be aggressive towards aquatic frogs) is that these can be attacked/eaten and they are all big messy fish. I guess if you had a huge tank it could work but there are other pitfalls as well, as others have mentioned (mainly disease).

I like the ghost shrimp idea. In my experience they last quite a while, yeah the frogs eventually hunt them down but it takes a few months for them to find every single shrimp (they are good at evading too!).

If you really want to go the fish route you could get guppies and breed them in another tank. They will be eaten, depending on the voracity of your frog it could take a hours to days to hunt down all the fish, some are less interested in fish though but most Xenopus will hunt relentlessly and are always hungry.